TM 55-1520-240-23-79-30ALTERNATING CURRENT (AC) POWER DISTRIBUTION(Continued)9-30THEORY OF OPERATIONThe four sources of ac power for the helicopter have thefollowing order of operation:NO BREAK - WORK HARDERWhen one or both engines are running, the No. 1 andNo. 2 generators supply power to their respective buses.If either generator fails or is shut down, that source willbe isolated from its bus and the remaining generatorwill then supply both buses through crosstie circuitry.With either or both of the main generators operating, theAPU generator and the external power receptacle aredisconnected from the buses and can supply no power.NO BREAK - WORK HARDERIf the APU is operating while both engines are shut downor both generators are manually switched off. The APUgenerator is automatically connected to both ac buses.The external power receptacle remains disconnectedfrom the buses.NO BREAK - WORK HARDEROnly if both main generators and the APU generator arenot operating is it possible to apply external power to thebuses. If, while an external power source is connected,any of the three generators is turned on, the externalpower source will be disconnected at the buses.NO BREAK - WORK HARDERIn all cases, the appropriate power source is connectedto the buses by power contactors, operated by 28 vdc.MAIN AC GENERATOR AND CONTROL UNITEach of the generators consists of three units: apermanent magnet generator (PMG), a main exciter androtor assembly, and the main generator stator windings.NO BREAK - WORK HARDERThe output of the PMG is fed to a generator control unit(GCU) in one of the electrical equipment compartments.Sensing circuits in the GCU average the three phasesof generator output voltage and apply this signal toan integral voltage regulator. The voltage regulatoruses this signal to regulate the current applied to thegenerator exciter and rotor section, keeping outputvoltage between 113 and 117 volts.NO BREAK - WORK HARDERIf generator output voltage drops below 102 voltsfor more than five to seven seconds, or exceeds122 to 125 volts for a period of time depending onthe magnitude of the overvoltage, relays within theGCU will shut down the generator and de-energizegenerator contactor 241 K1 in the PDP, disconnectingthe generator from the bus.NO BREAK - WORK HARDERSimilarly, if generator frequency drops below 330 Hz forthree to seven seconds, a relay within the GCU willdisconnect the generator from the system. However, thegenerator will continue to operate, and if frequency risesabove 335 Hz, it will automatically be reconnected tothe system.NO BREAK - WORK HARDERIf generator frequency drops below 320 Hz, thegenerator is disconnected from the bus and is also shutdown. To get the generator back on line after it has beenshut down, the generator control switch on the overheadELECT panel in the cockpit must be cycled from ON toOFF/RESET to ON.APU GENERATOR AND CONTROL UNITThe APU generator system is similar to that of eachof the two main generators. The generator suppliespower to the No. 1 and No. 2 ac buses when the maingenerators are not running. Like the main generators, itconsists of three units: a permanent magnet generator(PMG), an exciter generator, and a main generatorsection.NO BREAK - WORK HARDERThe PMG is used to monitor generator frequency forinput to the generator control unit. It supplies ac and dcpower as soon as the generator begins turning. Theac terminals (P1-PN) provide a minimum of 20 voltsat no load. The dc terminals (+ -) provide between 29and 40 volts.NO BREAK - WORK HARDERThe exciter generator provides main generator sectionexcitement and regulation. Terminals (F1 -A-) of theexciter section supply 3 to 14 volts during operation.NO BREAK - WORK HARDERThe main generator section supplies 115/200 volt, 400Hz ac output to the APU contactor in the No. 2 powerdistribution panel. Cross-tie to the No. 1 PDP is throughexternal power control relay 241K5 in the No. 1 PDP.9-93